Part II. On Aqueous Solutions. 67 
for there is superposed upon the normal variation of the suscepti- 
bility with the temperature (due to the change in the number of 
particle collisions), the effect of chemical association or disso- 
ciation. In fact, over such a range of transition the susceptibility 
is dependent upon a summation of terms for the classes of particles 
of definite types, the expression for each type containing a factor 
regulating the rate of association or dissociation at any particular 
temperature during the transition. 
On this view the nature of the continuity of the magnetic 
states of the so-called a, 8, y and 6 forms of iron is apparent. 
The theory of Weiss*, which assumes all the particles to be of 
equal size, cannot apply during the stage of transition. 
The rule proposed by Honda*+ in place of the Curie-Langevin 
laws is that the effect of a slight mcrease of the temperature on 
the susceptibility of an element is the same as that of a slight 
increase of the atomic weight. This rule is quite inadequate in 
the case of tin and many other elements, and even for those 
elements for which it does hold it is purely qualitative, and its 
application is therefore seriously limited. 
In what follows we shall assume (1) that the paramagnetic 
susceptibility of a substance composed of particles which do not 
vary in complexity with the temperature is inversely proportional 
to the absolute temperature, (2) that the diamagnetic suscepti- 
bility of a substance composed of such particles is independent of 
the temperature. 
(2) General Theory. 
Consider the solution of a salt in water. The salt will be 
ionised to an extent depending upon the concentration. Moreover, 
there may be present complex groups of molecules—groups of 
water molecules, hydrated ions and hydrated molecules of undis- 
sociated salt. (Recent work has shown that in strong solutions 
multiply complex salt molecules exist, hydrated to an unknown 
extenty.) 
We shall assume that the groups of associated molecules are 
unstable, and that whether they are composed of similar or dis- 
similar molecules, the effect of a change of temperature on the 
group is of a similar nature in the two cases. This implies that 
in the groups composed of dissimilar molecules, the nucleus, 
whether it be an ion or a salt molecule, merely acts as a centre 
round which a shell of associated molecules is condensed, while the 
nature of the forces of association is the same as it is for a group 
of similar molecules. 
Let there be NV types of particles. 
* Comptes Rendus, t. 144, p. 25, 1907. 
+ Comptes Rendus, t. 151, p. 511, 1910. 
+ Applebey, Journ. Chem. Soc., Trans. 11. p. 2000, Oct. 1910. 
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